The overall goal of this work is to understand bacterial chemotaxis as an integrated system of motor function modulated by sensory stimuli. Studies are underway to document with a high sensitivity video system the quaternary structural transitions which take place in the filaments and which participate in the directional changes which result in taxis. In another study we are attempting to describe the behavior of the flagellar motor as a function of the energetic state of the cell, to find the reason why reversibility of rotational sense is only possible at high power levels, and to determine whether flagella are under coordinate control.